1. Field of the Invention
The invention relates to mixer circuit for the very high frequency (VHF) television transmission bands I and III, which is switched dependent of the bands and which includes a mixer transistor, which operates via a collector resistor onto a two circuit intermediate frequency band filter, where the collector voltage is fed to the transistor via a band filter and a resistor.
2. Brief Description of the Backround of the Invention Including Prior Art
The mixer circuit set forth above is very common with television receiver circuits. Extremely interfering occurrences are found in band I both in Europe as well as in the United States, where the band I is expanded for CATV. These interferences are noted as Moire pattern on the screen. The band I designates a frequency region of from about 47 to 68 MHz and the band III designates a frequency region of from about 174 to 223 MHz. The channel Y is distributed in Cable Television grids and is associated with a picture carrier frequency of 76.25 MHz. In this context an interfering frequency of 37.35 MHz is generated as a mixer result. The signal of 37.35 MHz corresponds in the video region to an interfering frequency of 1.55 MHz, which can be seen as a Moire pattern.
From the equation EQU 2.times.BT-F.sub.OSC =F.sub.INTER
results that: EQU 2.times.76.25 MHz-115.15 MHz=37.35 MHz.
It can be recognized from this that the emitting station interferes with itself without any additional effects of other stations.
The channel 6 in the United States generates "colorbeat", which is also present in Band I as antenna "Off Air Info" and a self interfering Moire frequency of 0.75 MHz is present in the video region. From this is deduced EQU 2.times.BT-F.sub.OSC =F.sub.INTER EQU 2.times.83.25 MHz-124.5 MHz=42 MHz.
A band switching with the aid of switching diodes is known for example from the book by B. Morgenstern: Farbfernsehtechnik, 1977, B. G. Teubner, Stuttgart, Federal Republic of Germany, pages 134 and 135. However this does not represent a band dependent switching of a mixer.
It can be recognized from the German Patent Application laid open DE-OS No. 28 33 053, that the first basic part of the object of the reference agrees with the object of the present application, which is to eliminate Moire interferences. The object is recited on page 5, 2nd paragraph of the above patent application and states "in particular a channel selector is to be created, which is associated with a separating circuit for a peak value formation circuit, in order to avoid the generation of a "beat component". Therefore one starts in the recited patent application at the known wide band mixer according to FIG. 1 of the reference which is provided after the amplifier stage with the transistor 3 in emitter connection of peak value formation circuit 5 of the kind of a low pass filter. The low pass filter circuit comprises a capacitor 4, a coil 8 and a capacitor 9. This circuit 5 serves to supress the passage of an interference signal in the intermediate frequency circuit signal. The transistors 2, 3, 7 and the capacitor 9 are provided in the configuration of an integrated circuit in this known example, where only the capacitor 4 and the coil 8 are connected externally to the integrated circuits.
This mixer of FIG. 1 of the reference is associated with a damping characterizing line, which is designated as 14a in FIG. 4, which does not drop down abruptly enough at high frequencies, whereby the interfering signal component which has a higher frequency than the intermediate frequency band is not sufficiently damped in order to prevent the generation of a best interference occurrence by necessity.
This problem is resolved according to claim 1 of this reference patent application by employing a second capacitor 26 parallel to the inductivity L.sub.p for forming a separating circuit together with the inductivity. This secondary step according to FIG. 2 of the reference, which changes the load impedance /Z.sub.1 / for the transistor 23, generates also an alternating voltage effect and results in the generation of the curve course 15b of FIG. 5. It is too low for the intermediate frequency and the recited mixing frequency. It is a result of this that a large current flows in the transistor 23, which renders the dynamic region of the transistor 23 small and which forces a nonlinear operation in the transistor 23. Consequently, a beat component is generated. If this occurs in the transistor 23, than this interfering component, since it falls into the intermediate frequency band, is fed to the intermediate frequency amplifier 12 without damping via the peak value formation circuit 25, even if this circuit is provided with the recited separating circuit.
According to the circuit of FIG. 3 of the reference, a variant of the solution of the reference is provided which eliminates this defect by switching and connecting the capacitor 26 not directly parallel to the inductivity L.sub.p but by placing this capacitor in parallel to the base emitter section of the transistor 23. Advantageously in this context the capacitor 26 forms part of the integrated circuit. Thus the load impedance Z.sub.L takes on the shape of the line 15e in FIG. 5. Now the load impedancy Z.sub.L for the best frequencies is sufficiently large. Therefore the current flowing through the transistor 23 becomes small for the best frequencies and its dynamic region becomes large.
Thus according to this patent application in addition the control voltages of the generated beat frequencies are supressed depending on frequencies. The transistor 23 is blocked nearly completely in the region of the critical frequencies. Thus the solution according to reference 1 is basically secondary step by which a high voltage current impedance is generated for two of the best frequencies to be supressed.